Ancient Mammal Relatives Were 'Night Owls'

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Mammals were long thought to have evolved nocturnal lifestyles as
a way to co-exist with dinosaurs, but new research finds that
nighttime behavior may have evolved 100 million years earlier
than mammals did.

The eye bones of
ancestors of modern mammals that lived more than 300 million
years ago, such as the sail-finned carnivore Dimetrodon,
suggest that at least some of these animals were already active
at night, researchers say.

"Traditionally, dinosaurs were considered day-active, and mammals
were regarded as living in the shade of them, living at night,"
said Lars Schmitz, a biologist at Claremont McKenna, Pitzer and
Scripps colleges in Claremont, California, and co-author of the
study, detailed today (Sept. 3) in the journal Proceedings of the
Royal Society B. [ Gallery:
Evolution's Most Extreme Mammals ]

Approximately half of all living land mammals are nocturnal, and
many more are active at twilight, the researchers said. Humans
and other primates are some of the exceptions; they're active
primarily during the day. But it would be "a bit self-centered"
to assume diurnal behavior is the norm, Schmitz told Live
Science.

Mammals are part of a larger group, called synapsids, that
includes mammals and all extinct relatives that were more closely
related to living mammals than to birds, reptiles or amphibians.
The oldest synapsid fossils date back to 315 million years ago,
and the first mammals don't appear in the fossil record until
about 200 million years ago, the researchers said.

To investigate when
nocturnal behavior first evolved, Schmitz and his colleagues
from The Field Museum in Chicago examined some of the more
ancient synapsid fossils from museum collections in the United
States and South Africa. "We tried to look at bones that may be
related to their ability to see at night," Schmitz said.

Specifically, the researchers were interested in tiny, ringlike
bones in the eye known as scleral ossicles, which provide clues
to the size and shape of the eyes. Modern mammals lack these
bones, but they are found in the eyes of many other vertebrates.
The bones are very fragile, however, so they aren't often
preserved in fossils.

The researchers used a statistical technique to analyze the tiny
bones, comparing them with the bones of living animals to prove
that the method was robust.

The findings suggest that some synapsids were active during the
daytime and others during the nighttime, and some were active at
twilight. Interestingly, some of the oldest synapsids they looked
at, including Dimetrodon, had eyes whose size indicates these
animals were likely active at night.

The findings help researchers understand the biology of these
mammalian ancestors. "How were they living? How were they
dividing resources?" Schmitz said. "With these little puzzle
pieces, we can start building this image of what life may have
been like 250 million years ago."

Jörg Fröbisch, a biologist at Humboldt University of Berlin,
in Germany, who specializes in synapsid evolution, said he was
"very excited" about the findings. "All the evidence seems to
point to the fact that there was a wide variety of [daytime]
behavior" among these mammalian ancestors, said Fröbisch, who
knows the authors of the new study but was not involved in it.

Fröbisch's colleague at Humboldt, evolutionary biologist
Christian Kammerer, agreed."Mammals are thought to be ancestrally
nocturnal, but this study demonstrates that this was not some
crucial novelty in
mammalian evolution : There had been nocturnal members of the
mammalian stem lineage for a hundred million years," Kammerer
told Live Science.

However, Kammerer cautioned that the results should not be
interpreted as definitive, as the activity patterns in living
animals are "often messy," and animals may be active during both
day and night.

"The Field Museum shouldn't throw out its mural of a
Dimetrodon hunting by daylight quite yet," he said.